An optical transceiver transmits and receives light signals within a fiber optic network. The light source for the transceiver is typically an optoelectronic device, such as a Vertical Cavity Surface Emitting Laser (VCSEL), a Light Emitting Diode (LED), a laser, or other light-emitting device. The light output from the optoelectronic device is controlled by a driver circuit.
In digital transmissions, the driver circuit drives the optoelectronic device between high and low signal outputs to represent a digital data stream. The data is synchronized to a clock with a regular period. During a certain window of time during each clock cycle, the transmitted signal must clearly be a one or zero for the receiver to read the data correctly. Any signal transitions are done outside this window. When plotted on an oscilloscope, the transmitted signal looks like the graph in FIG. 1, known as an eye diagram. A transceiver with optimal performance has fast rise and fall times in its eye diagram without excessive ringing in the output.
The optoelectronic device and the driver circuit are typically formed as separate components on separate chips. There are several options for electrically connecting the optoelectronic device to the driver circuit. FIG. 2A shows one method, wherein a bond pad on the optoelectronic device 203 is connected to a bond pad on the driver circuit 205 with a conductive wire bond 207. FIG. 3A shows another method, known as a flip-chip configuration: the chip with the optoelectronic device 303 is flipped onto the chip with the driver circuit 305, and bond pads on the two chips are bonded together using solder balls 307. There are many other configurations for bonding the two chips together.
Since the data rate within optical networks is constantly increasing, the transitions between high and low outputs must occur faster and faster. However, the effect of parasitic capacitances and inductances within the transceiver becomes more pronounced as the transmission frequency increases, which negatively affects the performance of the transceiver.
For example, the wire bond connection 207 of FIG. 2A introduces inductance between the optoelectronic device 103 and the driver circuit 105, which changes the frequency response of the transceiver. Unfortunately, the size and shape of the wire bonds are not consistent: some wires may arc higher than others, and some wires may be shorter than others. Therefore, the inductance of each wire bond will vary as well. This variability lowers the manufacturing yield, because the frequency response of the transceiver may no longer meet specifications. FIG. 2B shows a sample eye diagram for an optical transceiver with excessive inductance in the wire bonds between the optoelectronic device and the driver circuit. The ringing effect in the transmitted signal makes it difficult for the receiver to determine whether the data is a one or a zero.
The flip-chip construction of FIG. 3A is also problematic. Since the solder balls 307 are relatively small, they introduce very little inductance into the system. However, a connection with too little inductance may also negatively affect the frequency response of the optical transceiver. FIG. 3B shows a sample eye diagram for an optical transceiver having too little inductance (which manifests itself in the signal as insufficient response peaking) in the wire bonds between the optoelectronic device and the driver circuit. The transmitted signal takes much longer to complete its transitions, which slows down the speed at which data can be transmitted.
Past attempts to compensate for these problems included redesigning the driver circuit, redesigning the bond pads, adjusting the length of the wire bond, or changing the arc of the wire. However, these methods are time-consuming, expensive, and difficult to repeat. Therefore, there remains a need for a way to electrically connect an optoelectronic device to a driver circuit to produce an optical transceiver with an optimal frequency response, and to do so with higher yield and better repeatability.